How should a glass material be setup in Cycles?
There are a couple things going on here:


The glass is not entirely white, so a noticeable amount of light is being absorbed. Setting it to white (or almost white) gives a more glassy result:



With a less uniform lighting/environment the glass is more visible:


In this render I adjusted the model to get rid of the subsurf artifacts and thicken the bottom a bit.
There is an air gap between the liquid and the glass. This isn't the way liquid normally sits in a glass in real life, so it might look funny. See Liquid / glass interface IOR and normals in Cycles.

If the interface between the liquid, air, ice, and glass are setup as described, you get a result like this: